1. Field of Invention
This invention relates to a method and apparatus for loading of test pins in a probe head (or adaptor) useful in automated testing of printed circuit boards and in particular to an automated probe loading method, and apparatus useful in that method.
2. Brief Statement of the Prior Art
The automated testing of printed circuit boards to detect manufacturing flaws and defects is commonly performed with a mechanical press with one or two test heads resting on the platens of the press. The test heads have many spring pins which are located at preselected points for testing of a printed circuit board with the most common pin-to-pin spacing being 0.1, 0.07 or 0.05 inch. The plates for the test heads are usually predrilled with a grid-like distribution of holes at the aforementioned uniform spacings.
Printed circuit board manufacturing capability, however, has advanced to very compact configurations, particularly with surface mounted components such as integrated circuits. These surface mounted components are mounted with connections which are at closer spacings than the 0.05 inch, pin-to-pin distances which are the closest that can be practically provided with a grid distribution of holes for test pins. Accordingly, there is an increasing demand for adapters which translate the geometrically uniform grid locations of spring test pins to off-grid locations. Probe heads, which comprise a plurality of thin wire probes are supported in a fixture formed of two or more insulating plates separated by spacers, have been used as adapters for this purpose.
A typical probe head comprises two or more insulating plates separated by spacers, usually at corner locations. The plate of the probe head which is placed on the test head of the tester has holes which are located "on-grid", i.e., at the geometrical regular spacings corresponding to the grid of the test head. The plate which will be adjacent the printed circuit board under test, has holes which are located at the precise test point locations of the circuit board under test, a number of which will be "off-grid", i.e., will not be at the precise location of a test pin in the test head. The probes, which are typically a straight length of stiff wire, e.g., piano wire, are manually loaded into the plates and, when the particular test point served by a probe is off-grid, the probe is canted at a slight angle to extend from the on-grid hole in the plate next to the test head to the off-grid hole in the probe plate which is next to the printed circuit board. Frequently one or more additional insulating plates are positioned at intermediate levels between the upper and lower plates of the probe head and holes for the probes are drilled to align with the holes in the upper and lower plates.
The task of manually loading many thousands of probes into a probe head is a tedious and costly operation. Each printed circuit has its own, unique pattern of test points, and a unique probe head must be provided for testing boards of each unique printed circuit. Often the printed circuit board carries printed circuits on both sides, and in such instances, two probe heads are required for double sided testing of those printed circuit boards. As a single probe head can contain as many as 10,000 probes, it is apparent that manual loading of the probes in probe heads is a very expensive and difficult task, and is a serious limitation on the automated testing of printed circuit boards.
Some manufacturers have attempted to automate the task of loading probes into probe heads. Shaker tables have been devised to load the probe heads automatically. The shaker tables have limited application however, as they cannot be used for the extremely closely spaced test point patterns such as experienced with surface mount components. In a conventional shaker table loading procedure, a probe head comprising two or more plates is placed on a shaker table, a plurality of probes are placed with their bottom ends resting on the top plate, and the entire assembly is vibrated to encourage the probes to seek holes in the plates and fall into their final positions.
Other manufacturers build pin loaders comprising an x, y positioning table and a pin inserting head. The x, y table moves each hole of the probe head beneath the pin inserting head which inserts a probe. This is a sequential operation which is very time consuming.